1. Field of the Invention
This invention relates to a method of making a semiconductor device, and more particularly to a method of forming polycrystalline semiconductor contact electrodes connected to diffused regions of a semiconductor substrate.
2. Description of the Related Art
Referring to the drawings, a conventional method of manufacturing contact electrodes connected to diffused regions of a semiconductor substrate will be described.
As shown in FIG. 5A, a semiconductor substrate 1 is prepared which has an N-type diffused region (e.g., an active region such as an emitter region) 3 and a P-type diffused region 4 exposed to its major surface. An insulating film (e.g., SiO.sub.2) 2 is formed on the substrate 1 by a suitable method such as CVD techniques. Contact holes 5 are then formed in the insulating film 2 by photolithography techniques. The diffused regions may be provided before making the contact holes, or after the contact holes have been made in the insulating film. Different conductivity type impurities may be diffused into the substrate to provide diffused regions therein. Next, as shown in FIG. 5B, on the insulating film including the contact holes 5, an Al alloy film 6 containing Si and/or Cu is deposited by sputtering or evaporation techniques.
With regard to such an electrode wiring film of the evaporated Al alloy film, if there is any thickness step on the substrate, the film thickness of that portion becomes nonuniform. Particularly, a large thickness step in the contact hole may often cause wiring layers to break or to increase the resistance, which reduces the reliability of the device. FIG. 5B illustrates a state called a step discontinuity of the electrode wiring layer within the contact hole. Such a step coverage problem becomes more significant as circuit elements have a fine structure.
With an increase in the integration density of LSIs, a doped polysilicon film has been used as a wiring material. As for a method of making the polysilicon film, thermal decomposition of SiH.sub.4 using normal-pressure or reduced pressure CVD techniques has been widely employed. FIG. 6 shows a process where after an undoped polysilicon film 7 is deposited instead of the Al alloy film shown in FIG. 5B, an impurity (e.g., As.sup.+ ions) is added by ion implantation techniques thereto.
When the ion implantation is used for doping of the impurity, however, it is difficult to uniformly dope the undoped polysilicon film therewith, resulting in an insufficient decrease in the resistance. When the impurity to be added to the polysilicon film 7 is formed of only one kind of N-type (As.sup.+), a p-n junction is formed at a portion 8 where the silicon film 7 is in contact with the P-type diffused region 4. Therefore, a good ohmic contact can not be provided. To avoid this, it is necessary to ion-implant an N-type impurity into the polysilicon film in contact with the N-type diffused region 3 and to ion-implant a P-type impurity into the polysilicon film in contact with the P-type diffused region 4, respectively. This requires two lithography processes for forming ion implantation masks, resulting in a longer processing time. Furthermore, heat treatment for activation after ion implantation is required which increases process steps. Accordingly, disadvantages such as an excessive spread of the diffused regions may occur.
On the other hand, when the diffused regions 3 and 4 of the semiconductor substrate 1 have been formed before contact holes are made, as shown in FIG. 7, overetching 5a in the substrate or a pattern misalignment 5b with the contact hole 5 can take place in making the contact holes, which may cause current leakage at the junction portions.
As described above, use of Al alloy film as the electrode wiring material causes the film thickness to be non-uniform at the thickness step. Particularly, the wiring layer may be broken at the contact holes due to the step discontinuity and the resistance of the wiring layer is increased.
In the case of the conventional technique of using polysilicon as the electrode wiring material and adding the impurity by the ion implantation, some problems of the step discontinuity in the Al alloy film are improved, but it is still difficult to uniformly dope a polysilicon film with the impurity, thereby sufficiently reducing the resistance of the contact electrode. In addition, when the conductivity type of the diffused region of the substrate differs from that of the polysilicon film connected to the region, an unwanted p-n junction is formed at the contact electrode. To avoid the problem creates another problem of increasing the number of process steps.